1. Field of the Invention
The present invention relates to a wavelength-dispersive X-ray spectrometer used in an electron probe microanalyzer (EPMA) or other similar instrument and, more particularly, to a technique for improving the performance of an X-ray spectrometer equipped with analyzing crystals curved in the direction of angular dispersion.
2. Description of Related Art
EPMAs are widely used as instruments for qualitatively and quantitatively analyzing a sample by sharply focusing an accelerated electron beam, directing the beam toward a surface of the sample, dispersing the generated characteristic X-rays, and analyzing the sample from the wavelengths and intensities of the dispersed X-rays. Generally, an EPMA is equipped with a wavelength-dispersive (WD) spectrometer designed to collect X-rays while moving the crystal along a straight path. This X-ray spectrometer may be hereinafter referred to as the WD spectrometer of the straight moving ray-collection type. Fundamental instrumentation of such an X-ray spectrometer is shown in the cross section of FIG. 1. When a focusing electron beam EB hits a sample 2, X-rays are produced. Electron optics for generating, accelerating, and focusing the electron beam EB are not shown.
An X-ray spectrometer 1 holds an analyzing crystal 3 whose center C moves on a straight line SC that is tilted at an angle of an X-ray takeoff angle α from a point of source S of X-rays. At this time, the point of source S, the center C of the analyzing crystal 3, and the center F of a slit 5 in an X-ray detector 4 are always present on the circumference of a Rowland circle 6 having a constant radius R. The position of the X-ray detector 4 and the center Q of the Rowland circle 6 move such that line segments SC and CF are kept equal in length. The curved crystalline lattice plane of the analyzing crystal 3 that extends along arc C2 always faces the center Q of the Rowland circle. The curved crystalline lattice plane is curved about a point D with a curvature of 2R. The point D is the intersection of an extension of a straight line CQ and the Rowland circle 6, the straight line CQ connecting the center C of the analyzing crystal 3 and the center Q of the Rowland circle 6. The length of the line segment SC is referred to as a spectral position L. Let θ be the angle of incidence of X-rays on the center C of the analyzing crystal. The angle θ is made between straight lines C1 and SC. The straight line C1 passes through the center C of the analyzing crystal and is tangent to the Rowland circle 6. The spectral position L is given byL=2R·sin θ  (1)Meanwhile, from the Bragg condition, the diffraction conditions for the analyzing crystal are given by2d·sin θ=n·λ  (2)where n is the order of diffraction and a positive integer, λ is the wavelength of X-rays, and d is the lattice spacing of the analyzing crystal. From Eqs. (1) and (2), we can obtain:
                    L        =                                            2              ⁢              R                                      2              ⁢              d                                ·          n          ·          λ                                    (        3        )            
It is possible to know the wavelength γ of the diffracted characteristic X-rays by measuring the spectral position L. Since the characteristic X-rays have a wavelength intrinsic to the element, the element contained in the sample can be identified. Furthermore, the concentration of the element contained in the sample can be known from the measured intensity of the characteristic X-rays.
Curved analyzing crystals have two types: Johansson type and Johann type. The differences between the Johansson and Johann types are shown in FIGS. 7(a) and 7(b) and FIGS. 8(a) and 8(b). FIG. 7(a) is a perspective view of a Johansson analyzing crystal, as viewed from inside a Rowland circle. First, the flat crystal is curved about a point D with curvature 2R such that the direction of angular dispersion of the analyzing crystal agrees with arc C2. Then, the curved crystal is polished with the same curvature R as the radius of the Rowland circle 6. Thus, X-rays incident on an arc of the analyzing crystal 3 in contact with the circumference of the Rowland circle 6 are diffracted while completely satisfying the requirement of Eq. (2) as shown in FIG. 7(b). However, the condition of Eq. (2) is satisfied less with going away from the arc in contact with the Rowland circle in a lateral direction perpendicular to the direction of angular dispersion. The double-dot-dash lines in FIG. 7(b) indicate positions with equal incident angle error. The double-dot-dash lines are referred to as equal incident-angle error lines. This tendency becomes more conspicuous with reducing the incident angle θ. Consequently, the wavelength resolution of the detected X-rays and the ratio of the intensity of the characteristic X-rays to the background intensity are deteriorated. Techniques for alleviating these problems are shown in Japanese Patent Laid-Open No. H10-239495.
The diffractive surfaces of Johansson crystals are physically polished. Therefore, some analyzing crystals for relatively long wavelengths have deteriorated performance and thus cannot be easily put into practical use. In this case, the following Johann type is used. FIG. 8(a) is a perspective view of an analyzing crystal in a Johann geometry, as viewed from the inside of a Rowland circle. In the Johann type, the direction of angular dispersion of the analyzing crystal is curved with curvature 2R about a point D such that the crystalline lattice plane extends along an arc C2. Under this curved condition, the crystal is used. In this type of analyzing crystal, X-rays incident on mutually crossing lines about the center C of the analyzing crystal are diffracted while completely satisfying Eq. (2) as shown in FIG. 8(b). Solid lines or dashed lines in FIG. 8(b) like the letter X expand in the direction of angular direction according to increasing the value of the incident angle θ. The double-dot-dash lines in FIG. 8(b) indicate positions with equal incident-angle error. The double-dot-dash lines are referred to as equal incident-angle error lines. The geometry of the mutually crossing lines varies with the value of L. As the incident angle θ decreases, the geometry approaches the center C of the analyzing crystal as shown as dashed lines in FIG. 8(b). Where it is difficult to polish the surface of an analyzing crystal or deterioration of performance with polishing should be avoided, a Johann geometry is used. LB (Langmuir-Blodgett) films often used as an analyzing element for X-ray spectroscopy for analysis of ultralight elements and analyzing elements using layered synthetic microstructures are difficult to polish and, therefore, they are used only in Johann geometry. Organic crystals synthetically produced from RAP (Rubidium acid phthalate), TAP (Thallium acid phthalate), or PET (Pentaerythritol) can be polished to make Johansson crystals, but they are often used to make Johann crystals because of a compromise with performance deterioration. A layered synthetic microstructure is created by artificially stacking a layer of high X-ray scattering capabilities and a spacer layer for securing lattice spacing on a substrate alternately. This microstructure is also referred to as an artificial superlattice. Analyzing elements of LB films and layered synthetic microstructures are not crystals in proper meaning but they are herein conveniently referred to as analyzing crystals.
An analyzing crystal is curved such that larger parts of X-rays emitted from a point X-ray source S are diffracted. However, both Johansson and Johann crystals of FIGS. 7(a) and 7(b) and FIGS. 8(a) and 8(b), respectively, are curved in only the direction of angular dispersion. In this case, the opening of the slit 5 in the X-ray detector 4 needs to have a length of 2W in a direction parallel to the widthwise direction of the analyzing crystal 3 as shown in FIG. 9. However, there is the problem that spatial restrictions are inevitably imposed when a wide slit is placed. Especially, the Johann analyzing crystal is affected greatly by limitation on the length of the slit, because the completely diffracted region is an X-shaped form and thus the width of the analyzing crystal can be increased with desirable results. In an attempt to avoid this problem and to obtain a high-intensity X-ray spectrometer, a two-directional curved analyzing crystal that is curved even in the widthwise direction of a Johann analyzing crystal has been fabricated. A two-directionally curved analyzing crystal having spherically-curved concave surfaces both in the direction of angular dispersion of the curved analyzing crystal and in a direction perpendicular to the direction of angular dispersion is herein referred to as a spherically-curved, Johann-type analyzing crystal, the concave surfaces having the same curvature as the diameter of the Rowland circle.
In a curved analyzing crystal fitted to an X-ray spectrometer mounted in an EPMA, the effective diffraction area actually contributing to diffraction differs depending on whether it is a Johansson or Johann crystal, on the spectral position L, and on the kind of analyzing crystal used. In some cases, the effective diffraction area is only about a half of the total area of the analyzing crystal.
The aforementioned spherically-curved Johann analyzing crystal has an optimum angular dispersion direction length according to the wavelength of the selected X-ray. That is, the length in the direction of angular dispersion is relatively small for shorter wavelengths of X-rays. The length in the direction of angular dispersion is relatively large for longer wavelengths of X-rays. Therefore, a spherically-curved, Johann-type analyzing crystal fabricated to match the length suitable for one wavelength of characteristic X-rays of interest within the analyzed range cannot be suitably used for spectral analysis of other characteristic X-rays which are widely different in wavelength from the X-ray to be selected. For example, the spectral waveform of the characteristic X-rays at wavelengths shorter than the X-rays to be spectrally selected has a tail on the lower diffraction angle side (on the shorter wavelength side), deteriorating the wavelength resolution. In very bad cases, lumpy hills appear on the waveform. This may impair the reliability of the waveform itself. Furthermore, there is the problem that the total area of the analyzing crystal is narrower than the effective diffraction area for characteristic X-rays longer than the X-rays to be spectrally selected, giving rise to a loss of the detectable X-ray intensity.
In an ordinary curved crystal, there is the problem that X-rays enter even those portions which do not contribute to diffraction, deteriorating the wavelength resolution of the detected X-rays and the ratio of the intensity of the characteristic X-rays to the background intensity. In an attempt to solve this problem, Japanese Patent Laid-Open No. S52-27695 discloses a technique using a disk having various sizes of X-ray takeoff windows between a source of X-rays and an analyzing crystal. An operator can select an X-ray takeoff window matched with the effective diffraction area by manipulating the disk from outside the vacuum. However, it is not easy for the operator to select an X-ray takeoff window of appropriate size. Consequently, there is the problem that it is laborious to switch the X-ray takeoff window by manual manipulations. Furthermore, it is impossible to cope with continuous variation of X-ray wavelength.